Knuth–Morris–Pratt Algorithm
Knuth–Morris–Pratt (a.k.a KMP Algorithm) is a string search algorithm. it searches for occurrences of a sub-string within a main-string by employing the observation that when a mismatch occurs, the word itself embodies sufficient information to determine where the next match could begin, thus bypassing re-examination of previously matched characters.
int* AddItemInArray(int** arr, int count, int item) {
int* newArr = malloc(sizeof(int) * (count + 1));
for (int i = 0; i < count; i++) {
newArr[i] = arr[i];
}
newArr[count] = item;
return newArr;
}
void ComputeLPSArray(char* pat, int m, int* lps)
{
int len = 0;
int i = 1;
lps[0] = 0;
while (i < m)
{
if (pat[i] == pat[len])
{
len++;
lps[i] = len;
i++;
}
else
{
if (len != 0)
{
len = lps[len - 1];
}
else
{
lps[i] = 0;
i++;
}
}
}
}
int* SearchString(char* str, char* pat) {
int* retVal = malloc(sizeof(int));
int retValSize = 0;
int M = strlen(pat);
int N = strlen(str);
int i = 0;
int j = 0;
int* lps = malloc(sizeof(int) * M);
ComputeLPSArray(pat, M, lps);
while (i < N)
{
if (pat[j] == str[i])
{
j++;
i++;
}
if (j == M)
{
retVal = AddItemInArray(retVal, retValSize++, i - j);
j = lps[j - 1];
}
else if (i < N && pat[j] != str[i])
{
if (j != 0)
j = lps[j - 1];
else
i = i + 1;
}
}
free(lps);
return retVal;
}
Example
char* data = "the quick brown fox jumps over the lazy dog";
int* value = SearchString(data, "the");
Output
0
31